Dose 2016

TPS dose engines: common misunderstandings

Why do some TPSs require CT calibration tables in terms of relative mass density e.g. in point kernel dose calculation engines it is TERMA and point kernels that are scaled ( ) ( ) ( ) ( ) ( ) ∫ Ψ ≈ ρ µ − r dr r r medium rMV o e r medium rMV rT 0 ' ' ' ,' , , ,

ρ µ

Primary energy fluence at the surface

Effective (spectrum averaged) mass attenuation coefficient at radiological distance r for the medium with certain density

• Mass attenuation coefficients usually pre-stored as weighted averages for an energy spectrum and for media with different composition and mass density • N CT ⇒ relative mass density and material composition • ⇒ mass attenuation coefficient and linear attenuation coefficient

e.g. in Pinnacle, TomoTherapy PS